The relationship between the thermohaline circulation and climate variability

Abstract

The long-term integration with the Global Ocean-Atmosphere-Land System model of the State Key Laboratory of Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics(IAP), Chinese Academy of Sciences has been used in the investigations on the relationship between the thermohaline circulation and climate variability. The results show that the strength of the North Atlantic Thermohaline circulation (THC) is negatively correlated with the North Atlantic Oscillation (NAO). Based on this kind of relationship, and also the instrument- measured climate record such as air pressure and sea surface temperature, the activity of the thermohaline circulation during the 20th century has been evaluated. The inferred variations of the strength of the THC is that, during two multi-decadal periods of 1867 1903 and 1934 1972, the THC is estimated to have been running stronger, whereas during the two periods of 1904 1933 and 1973 1994, it appears to have been weaker. The climate system of the Earth can be broken down into a number of components: the ocean, the atmosphere, the lithosphere, the cryosphere, and the biosphere. The ultimate forcing mechanism of oceanic and atmospheric motions is the pole-to-equator gradient of radiative heating of the planet. There exists surplus of heat in the equatorial region and deficit of heat in the polar region. In order to keep the thermal balance of the Earth, the excessive heat collected in the equatorial regions must be transported to the polar regions. The transport is taken up by the meridional circulation branch of the climate system, which is mainly made up of the atmospheric components of the famous Hadley cell, the Ferrel cell, and the polar cell, and also its ocean component of the Atlantic thermohaline circulation. About 90% of the water in the world ocean are involved in the thermohalien circulation, which is located beneath the wind driven surface currents and at great depths. The power for this slower, deeper circulation comes from the action of gravity on adjacent water masses of different densities. Since density is largely a function of temperature and salinity, the circulation due to density difference is called thermohaline circulation. The contemporary thermohaline circulation is characterized as the Atlantic Great Ocean Conveyor Belt. Intense radiative cooling of the polar ocean surface during the winter causes the formation or extension of sea ice. Cooling and brine rejection (i.e. salt ejection during ice formation) produces an unstable density gradient in the upper ocean allowing the cool and saline dense water to mix down convectively. Just as the tropical ascent is the rising part of the atmospheric direct circulation (i.e. warm air rising), the sinking motion of the ocean is the thermally direct part of the ocean circulation (i.e. cold water sinking). The thermodynamically direct circulation converts potential energy into kinetic energy. These processes lead to the formation of the deep ocean water, which spreads out away from the poles until it slowly ascends toward the equator. At last, the water moves back to the poles from the equator in the form of shallow warm water circulation (1,2) .